The invention relates to a method for analysing gases according to the counterflow principle, comprising a mass spectrometer tube located at the suction side of a molecular pump, both a primary vacuum pump and an inlet for the gases to be analysed being connected to the outlet of the molecular pump.
Because of the ionisation processes used, gas analysis methods employing mass spectrometers suffer from serious defects, which strongly reduce or even render impossible their applicability to several problems. These defects include the molecule fragmention during ionisation, which complicates the detection of the initially present molecules, because almost identical fragments can result from very different materials.
During ionisation, electrons are generally brought to a kinetical energy of at least 70 eV. At this energy level, the effective cross-sections and the ionisation efficiency present useful values, but the molecules are then fragmentated, which constitutes a decisive drawback. For example, the ionisation of carbon dioxide and carbon monoxide by electrons results in practically identical fragments and renders the separate detection of these two gases difficult.
Within certain limits, good results can be achieved by not only evaluating the main peak of the mass spectrometer, but the entire spectrum including the lateral peaks caused by the fragments, which signifies a so-called mass spectrometer fingerprint (see for example the periodical Vakuum 16, 67 (1966) "The interpretation of mass spectra in vacuum measurement"). However, many of the fingerprints are so similar, that their distinction becomes impossible. Thus, for example, the main peak of n-butane (mass 58) is located at mass 43 and the next following peak at mass 29, while acetone shows a main peak also at mass 43 and one of the secondary peaks at mass 29.
In order to avoid these difficulties in the interpretation of spectra, it has been proposed to proceed to a so-called soft ionisation, in order to reduce the creation of fragments as far as possible. Soft ionisation signifies a ionisation in two steps, the first step being constituted by the creation by electrons of a primary ion cloud of an inert gas. The primary ions are then mixed to the gas to be analyzed and, by means of charge transfer reactions, the different molecules of the gas to be analysed are converted into ions. The appropriate choice of primary ions having a ionisation potential, which is only slightly higher than that of the gas to be ionized, allows the neutral molecules to be transformed into ions without substantial fragmentation.
However, this method is too complicated for routine operations.